TxtStream SMS Messaging Service; add message "overhead" for New Messaging Interface

For TxtStream in New Zealand, using a short code requires a 9 character "signature" added to the message - to advise the cost of replies. This is automatically added by TxtStream.

To handle this, Cost Centre options for TxtStream now include a special "Message Overhead" setting. You can define the number of characters to allow for overhead.

The length of the message, message parts etc will factor this in and provide feedback to the User.

WARNING: This change only applies to the New Messaging Interface. If you currently use the Old Messaging Interface we strongly recommend reviewing whether you should switch to the new Messaging Interface.